The present invention relates to a method of measuring a rotational speed of a wheel of a motor vehicle and to an apparatus for measuring a rotational speed of a wheel of a motor vehicle.
Such methods and apparatuses are known from the background art. For control of recently designed brake systems or traction control systems that is tuned to the actual vehicle operating state, the rotational speed of individual wheels of a motor vehicle has to be monitored permanently with sufficiently high precision. For this purpose, speed sensors of various designs, such as for example inductive sensors, Hall sensors or the like, are used. Such speed sensors generally supply a pulsating output signal with offset, which ideally ranges between only two current values, a low current value also known as “low” current, which is for example ca. 7 mA, and a higher current value, also known as “high” current, which is for example ca. 14 mA. The “low” current arises, for example, when the sensor is not measuring a signal, whilst the “high” current arises when the sensor is measuring a signal. Ideally, i.e. in the case of measurement with high-frequency sampling, a distribution of sampled values that very precisely reproduces the actual characteristic of the output signal is obtained, wherein signal edges are also measured. With the aid of statistical formulations, it is then possible to determine the sampled values, the amplitudes of which occur most frequently. The value that occurs most frequently is then also referred to as “mode”.
It has however emerged that during operation of a motor vehicle the rotational speed measurement may be adversely affected by various disturbing influences. For example, an unwanted series resistance in the sensor line, short circuits or components of impaired function may lead to a displacement of the signal level of the output signal. As a result of this, the decoding electronics are no longer able to identify the signal level correctly and after further processing possibly transmit false digital pulses. The brake system or traction control system would consequently use false wheel speeds, thereby possibly leading to a malfunction. In order to be able to identify such errors, from the output signal in each case both current levels, namely the “low” current and the “high” current, have to be measured in order to ensure that the decoding electronics are able to convert these signal levels correctly into digital signals. However, as the output signal is a frequency signal, according to Shannon's sampling theory the sampling frequency would have to be at least twice as high as the highest frequency occurring in the signal to be sampled. This would result in a sampling frequency that demanded a considerable computing capacity of a processor. Corresponding processors are cost-intensive to acquire and therefore add undesirably to the cost of the total system.
From DD 251 003 A5, and corresponding U.S. Pat. No. 4,807,164, both of which are incorporated by reference herein, an apparatus for measuring a rotational speed of a wheel of a motor vehicle is known. This apparatus comprises a sensor for measuring the actual rotational speed and produces a corresponding output signal. A sampling device samples the output signal of the speed sensor at a predetermined sampling rate per measuring cycle. A signal processing device processes an output signal sampled by the sampling device and determines the amplitudes of the sampled values obtained in the measuring cycle. The sampled values are then correlated, with the result that a function providing information about the actual velocity of the moving body is obtained.